1. Field of the Invention
This invention relates to variable displacement piston pumps and particularly to pumps of the type described which are compensated to maintain constant pressure with variable flow. More particularly, this invention relates to a pump wherein the shaft assembly can rotate without dissipating energy by stirring hydraulic fluid.
2. Description of the Prior Art
A type of variable displacement piston pump known in the prior art, includes a shaft having a driven end and an opposite end arranged for supporting a swash plate. The swash plate is caused to pivot or slide about a fixed axis displaced from and generally perpendicular to the center line of the driven shaft. The swash plate is disposed in a cavity filled with the hydraulic fluid to be pumped. A plurality of cylinders having pistons disposed therein are arranged with associated check valves in a fixed pump block. During the delivery stroke of the pistons, pressure in the cylinders becomes high enough to open the check valves and deliver fluid to a common discharge manifold. When the manifold pressure approaches a predetermined set value, a force is created which is transmitted to the swash plate and slidingly pivots the swash plate about the fixed axis, away from a maximum flow position. The pistons are arranged with respect to the swash plate so that when the swash plate pivots away from the maximum flow position, the stroke of the pistons are decreased to reduce fluid flow and pressure. Equilibrium is thus established and reduced fluid flow at a predetermined substantially constant pressure is maintained. A Prior art example of such a pump is described in U.S. Pat. No. 4,149,830 having Frank Woodruff as the named inventor.
In the pump disclosed in U.S. Pat. No. 4,149,830, a plurality of piston assemblies are permitted to reciprocate in a cylinder block which is fixed to the pump casing. The swash plate, in addition to pivoting to achieve equilibrium is caused to rotate. The rotation of the swash plate forces the pistons to reciprocate, thus achieving the desired pump action. The drive shaft and the attached swash plate form a rotating assembly.
In aircraft hydraulic systems, it is important to maintain high efficiencies in order to minimize weight and power requirements. In prior art pumps, the rotating assembly is typically submerged in hydraulic fluid, and therefore must rotate and stir that fluid as the pump operates. This prior art construction reduces pump efficiency and therefore necessitates more input power.
In some prior pumps an arcuate cutout in the pump's shaft assembly supports the swash plate. This forms a sliding interface associated with varying the displacement of the pump. The frictional resistance to the onset of this sliding action causes a delay in the desired pivoting motion of the swash plate. Unwanted discharge pressure fluctuation may thus be caused by the swash plate's resistance to pivoting. It is desirable in aircraft applications to provide a high pressure variable output pump which is light in weight and has a smooth output.